1. Field of the Invention
The present invention relates to methods for making elements having microirregularities. In particular, the present invention relates to a method for making an optical element capable of correcting the shape of a diffraction grating to improve for low diffraction efficiency of the diffraction lattice which is caused by production errors, such as misalignment of masks.
2. Description of the Related Art
Since a combination of diffraction gratings and refracting elements has some advantages, for example, correction of chromatic aberrations and lightweight optical systems, it has been intensively studied for practical use in various fields. In typical methods for making diffractive optical elements in recent years, a blazed pattern on the surfaces of diffractive optical elements is replaced with an approximated multilevel pattern, and the elements are produced using lithographic processes used in the production of semiconductor devices. These methods facilitate the precise production of fine-pitch configurations, and have been applied to commercial production in a variety of applications.
Diffractive optical elements having multilevel surfaces are referred to as binary-type diffractive optical elements. An ideal blazed element has a diffraction efficiency of 100%, whereas an approximated multilevel element having N levels or steps has a diffraction efficiency represented by equation (1):
{sin(xcfx80/N)/(xcfx80/N)}2xc3x97100(%)xe2x80x83xe2x80x83(1) 
For example, when N=8 in equation (1), the efficiency is 95%, which is a sufficiently practical level for ordinary optical systems. A higher diffraction efficiency, for example, 99% or more, is achieved by increasing the number of levels to 16.
FIGS. 5(1) to 5(12) are cross-sectional views of a conventional method for making a diffractive optical element. In general, to form 2N levels, some steps are repeated N times using N masks, and in FIG. 5, a diffractive element having a pitch P of 8 (eight levels) is produced by using three masks. FIGS. 5(1) to 5(4) are steps for forming a two-level repeating unit using a first mask 3, FIGS. 5(5) to 5(8) are steps for forming a four-level repeating unit using a second mask 5, and FIGS. 5(9) to 5(12) are steps for forming an eight-level repeating unit using a third mask 7.
With reference to FIG. 5(1), a positive-type resist 2 is applied to a transparent glass substrate 1 having a refractive index n, and the pattern of the first mask 3 is transferred onto the resist 2 by exposure light L. The pattern of the first mask 3 has a pitch P, and the width of both the shading section and the open section is P/2. Next, the resist 2 is developed in FIG. 5(2) and the glass substrate 1 is etched in FIG. 5(3). The residual resist is removed to form a two-level repeating unit, as shown in FIG. 5(4). The required etched depth d1 is represented by the equation (2):
d1=(xcex/2)/(nxe2x88x921)xe2x80x83xe2x80x83(2) 
wherein xcex is the operating wavelength.
Next, a resist 4 is applied to the glass substrate with the two-level repeating unit and is exposed using the second mask 5, as shown in FIG. 5(5). The pattern pitch of the second mask 5 is half of the pattern pitch of the first mask 3, and the shading section and the open section both have a width of p/4. For exposure, the pattern edge is exactly aligned to the edge of the two-level configuration. With reference to FIG. 5(6), a resist pattern is formed after development. With reference to FIG. 5(7), the glass substrate is etched by a second etching process and the residual resist is removed. As shown in FIG. 5(8), a four-level repeating unit is formed on the glass substrate. The etched depth d2 by the second etching process is represented by equation (3):
d2=(xcex/4)/(nxe2x88x921)xe2x80x83xe2x80x83(3) 
Finally, an eight-level repeating unit is formed. A resist 6 is applied to the substrate having the four-level surface and is exposed through the third mask 7, as shown in FIG. 5(9). The pattern pitch of the third mask 7 is half of the pattern pitch of the second mask 5, and the shading section and the open section both have a width of P/8. For exposure, the pattern edge is exactly aligned to the edge of the four-level configuration. With reference to FIG. 5(10), a resist pattern is formed after development. With reference to FIG. 5(11), the glass substrate is etched by a third etching process and the residual resist is removed. As shown in FIG. 5(11), an eight-level repeating unit is formed on the glass substrate. The etched depth d3 by the third etching process is represented by equation (4):
d3=(xcex/8)/(nxe2x88x921)xe2x80x83xe2x80x83(4) 
An eight-level repeating unit having a pitch of P and a height per step of d3 is thereby produced, as shown in FIG. 5(12).
FIG. 6 shows the relationship between the three masks 3, 5 and 7 and the relative positions in the final eight-level repeating units. The difference between the highest level and the lowest level is (7xcex/8)/(nxe2x88x921) and the height per level is (xcex/8)/(nxe2x88x921).
In this case, the maximum diffraction efficiency for primary diffracted light is 95%. When quartz (SiO2) with a refractive index n of 1.51 is used as the substrate, the height per level is 61 nm for xcex=248 nm.
However, an eight-level binary diffractive element has a diffraction efficiency of 95% only when Fresnel reflection between air and the substrate neglected. The theoretical diffraction efficiency of the eight-level element becomes approximately 91% when the optical loss due to the Fresnel reflection at the boundary is taken into account.
In the above conventional method, however, misalignment between the plurality of masks causes problems. In FIG. 6, if misalignment does not occur between the three masks 3, 5 and 7, ideal eight-level repeating units are formed. It is, however, difficult to avoid misalignment between the masks 3, 5 and 7 in actual production processes, and thus such misalignment will inevitably occur.
FIG. 7 is an exemplary surface shape of an element when misalignment occurs between the three masks 3, 5 and 7, in which the first mask 3 and the second mask 5 are exactly aligned, but the third mask 7 is misaligned. The surface profile changes by such misalignment between these masks 3, 5 and 7, and generally has a complicated pattern having microirregularities rather than an ideal eight-level pattern.
The surface profile shown in FIG. 7 is obtained by calculation under the assumption that the substrate is ideally etched only in the vertical direction. The narrow protruding sections, however, also are etched during the etching process. In addition, the etching rate is not uniform over the entire substrate. As a result, for example, a diffractive element having a pitch P of 5.0 xcexcm has a surface profile as shown in FIG. 8, according to scanning electron microscopy. The diffraction efficiency of this element is at most 79%, which is significantly lower than the ideal value of 91%.
It is an object of the present invention to provide a method for making an optical element without misalignment of masks which results in a decrease in diffraction efficiency.
According to a first aspect of the present invention, a method for making an element includes forming an irregular pattern on a substrate, and forming a film for correcting errors in the irregular pattern.
Preferably, at least one antireflective film having a different refractive index from that of the substrate is formed.
According to a second aspect of the present invention, a method for making an optical element includes forming a multilevel pattern on a substrate by a lithographic process, and forming a film having a refractive index which is the same as that of the multilevel pattern.
The lithographic process preferably includes one of an etching process and a deposition process.
According to a third aspect of the present invention, a method for making an optical element comprises repeating a plurality of times a series of steps including a step of applying a resist to a substrate, a pattern transfer step, an etching step and a resist removal step to form a multilevel element pattern on the substrate; and forming a film having a refractive index which is the same as that of the multilevel element pattern.
According to a fourth aspect of the present invention, a binary-type diffractive optical element is produced by any one of the above-described methods.
Preferably, the film has a thickness which is one half to three-quarters the height of a level in a diffraction grating constituting the binary-type diffractive optical element.
According to a fifth aspect of the present invention, an optical system includes a diffractive optical element produced by one of the above-described methods.
According to a sixth aspect of the present invention, an exposure apparatus includes the above-described optical system.
According to a seventh aspect of the present invention, a method for making a device includes transferring a circuit pattern onto a wafer using the above-described exposure apparatus.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.